A method of this type is known from U.S. Pat. No. 5,053,774. When carrying out this method, as responder a transponder is used which does not contain its own energy supply source but which generates the energy necessary for its operation from the HF interrogation pulse transmitted by the interrogation device and received by said responder through an antenna. The antenna of the transponder used as responder device is part of a resonant circuit of which the resonant frequency is tuned exactly to the frequency of the interrogation pulse. Since the interrogation device transmits the HE interrogation pulse with an exactly defined frequency, the resonant circuit of the transponder must be tuned exactly to said frequency to enable enough HE energy to be received for generation of the supply voltage in the transponder. If the resonant circuit of the transponder is out of tune with the frequency of the HF interrogation pulse or is not correctly adjusted, only an inadequate transfer can take place from the interrogation device to the transponder and as a result the supply voltage generated in the transponder might possibly no longer suffice for carrying out the desired sending back of tile data stored therein.
The data stored in the transponder serving as responder device may consist for example of an identification number so that by said number for example an article to which the transponder is attached can be identified. It is also possible to implant the transponders into animals so that the latter can then be uniquely identified by the numbers during their entire life. However, there are uses in which due to the immediate environment of the transponder it cannot be ensured that the resonant circuit retains its desired nominal frequency exactly. If the article to which the transponder is attached is for example an aluminium case such as the type used in luggages the resonant frequency of the resonant circuit in the transponder is shifted to higher frequencies whilst at the same time the quality of the resonant circuit decreases. This detuning of the resonant circuit has two disadvantageous consequences. Firstly, the energy transmission from the interrogation device to the respond device by means of the HF interrogation pulse is impaired because the oscillating circuit in the responder device no longer operates in resonant mode. If the energy transmitted still suffices to initiate the sending back of the stoned data, then said sending back is done at a frequency which does not correspond to the desired frequency of the interrogation device . This can lead to the data no longer being uniquely interpreted in the interrogation device.